It is estimated that in many countries the number of used tires produced per year is approximately equal to the population of the country. As an example there are more than 250 million used tires produced annually in the United States. Methods of dealing with these used tires can generally be placed in two categories; disposal and reclamation. The former group includes land filling and stock piling which are increasingly unacceptable options for a multitude of reasons. Within the latter group are approaches that use the tires in close to their original state with possibly some physical processing. Examples of uses within the above group include use as vibration and debris dampening mats as may be used in drilling operations or filler material for road construction and burning as a source of energy. Burning has at times and in certain areas represented up to 40% of the tires being discarded. Most of the applications in the above group represent a limited volume of tires and do not exploit the economic value imparted on the raw materials during the original fabrication of the tire.
Another group of reclamation methods look to extracting increased value from the constituent materials within a tire. The major constituents include synthetic and natural rubber, carbon black and steel and minor constituents include sulphur and any stabilizers. The processes within this group may be referred to as reduction processes where the tire is being reduced to its constituents.
Reclamation of natural rubber using water, and more particularly steam, is known in the art. It has been disclosed that natural rubber can be reclaimed by processing with steam at temperatures above 100° C. The disclosed pressures include the saturated water vapour pressure. The disclosed methods provide for the devulcanization of natural rubber and possibly some depolymerization, depending on the particular reaction conditions.
It however became common practice to reclaim natural rubber at temperatures around 200° C. and pressures of around 200 psi. It was subsequently determined that synthetic rubbers could not be reclaimed using these latter conditions. D. S. LeBeau, discusses the reclamation of rubber that includes styrene-butadiene rubber (SBR), a synthetic rubber, using steam in Science and Technology of Reclaimed Rubber, Rubber Chemistry and Technology 40, 1967, 217–237. FIG. 1, FIG. 1 of LeBeau, illustrates effect of steam treatment on natural rubber and SBR. Natural rubber softens i.e. the viscosity is lowered and can be reclaimed when exposed to 200 psi steam. However, SBR experiences a short-lived softening that is followed by an extended hardening. LeBeau notes that the rate of hardening increases with increasing temperature. LeBeau further notes that the reclamation of synthetic rubbers therefore requires reclaiming agents or catalysts.
One process used for the reduction of used tires that include synthetic rubbers is pyrolysis. In a typical pyrolysis process the tires are subjected to temperatures between 600 and 900° C. in either an inert atmosphere or a vacuum. This process produces light oils and char where the char contains carbon black and pyrolytic carbon formed by the carbonization of rubber hydrocarbon.
Pyrolysis is generally not seen as a desirable reduction process. With the rubber hydrocarbons being either reduced to light oils or carbonized to char the end products of a pyrolysis process do not retain much of the economic value associated with the original rubber hydrocarbon and carbon black.
Reduction processes typically include devulcanization and depolymerization steps or processes. The devulcanization process breaks sulphur-sulphur and sulphur-carbon bonds that cross-link rubber molecules. The devulcanization process produces a solid residue where the mass of the solid residue is approximately 100% of the original mass of tire. The solid residue contains rubber hydrocarbon and carbon black where rubber hydrocarbon includes any hydrocarbon with a molecular weight above that of oil that originates from the initial rubber. The rubber hydrocarbon has an average molecular weight that is generally less than the initial rubber but much greater than oil, where oil has an average molecular weight of approximately 500 or less.
The depolymerization process reduces the average molecular weight of the rubber hydrocarbon by breaking carbon-carbon bonds of the rubber hydrocarbon until, at completion, the rubber hydrocarbon has been reduced to oil. Thus the depolymerization process, at completion, reduces the molecular weight from around 200,000 to 500. At the end of the depolymerization process the mass of solid residue is approximately 40% of the initial mass of tire. At this point the solid residue is substantially only carbon black with the rubber hydrocarbon being completely reduced to oil.
FIG. 2a is schematic graph of the % completion v. time for a typical pyrolysis process. In a typical pyrolysis process a devulcanization process 202 and a depolymerization process 204 occur substantially in parallel. As such the two processes are complete at approximately the same time i.e. tv≈tp. FIG. 2b shows a non-pyrolysis process in which the devulcanization process 206 is separated from the depolymerization process 208. At time tv the devulcanization process 206 is complete while the depolymerization process 208 is only a fraction of the way to completion.
An alternative approach for reducing tires uses solvent extraction techniques. Solvent extraction uses elevated temperatures and pressures in the presence of a solvent to at least devulcanize and often depolymerize the rubber. In almost all of the work in this area processing is conducted at a temperature and pressure that are above the critical values of these parameters for the particular solvent in which processing is being conducted.
Supercritical reaction conditions are defined as having a reaction temperature that is above the critical temperature and a pressure that is above the critical pressure. Supercritical reactions have been performed using a variety of solvents including alcohols, organic solvents and water. Much of the work using supercritical reaction conditions has been directed to extensive depolymerization of the rubber. In particular work has been directed to the reduction of the rubber to oil. Supercritical processing has been found to be advantageous in these cases as it provides a feasible reaction rate for the required depolymerization reactions. However, as the resulting oil will generally be used as a fuel the economic value of the rubber is reduced to a level well below that of that imparted to the tire during initial processing.
Recently there has been work directed to the devulcanization of rubber while mitigating depolymerization of the devulcanized hydrocarbons. By maintaining the hyrodrocarbon chain length near its original value a higher proportion of the economic value imparted to the rubber during initial processing is maintained. U.S. Pat. No. 6,548,560 to Kovalak et al. discloses the use of subcritical processing with a solvent selected from alcohols and ketones while U.S. Pat. No. 5,891,926 to Hunt et al. discloses subcritical processing conditions with the use of 2-butanol as a solvent. The use of the above solvents allowed processing temperatures below 300° C. which Kovalak et al. teach as important in reducing the amount of polymer degradation. Thus their focus is on solvents that have a critical temperature between 200 and 350° C. Hunt et al. and Kovalak et al. do not however disclose complete devulcanization of the rubber.
Many organic solvents are however costly and have properties that make them less than desirable with regard to health and safety considerations. For example, 2-butanol is flammable, has a low flash point and is an irritant.